Various types of variable compression ratio mechanisms are conventionally known, which change mechanical compression ratios of internal combustion engines. For example, the present applicant has proposed many variable compression ratio mechanisms of the type capable of moving up and down the top dead center position of a piston by changing the link geometry of a multi-link type piston-crank mechanism. There are also known variable compression ratio mechanisms of the type capable of changing a mechanical compression ratio by moving up and down the position of a cylinder relative to the center position of a crank shaft.
It is basically preferable for the variable compression ratio mechanism to set the compression ratio as high as possible within the range that does not cause abnormal combustion such as knocking. Thus, the target compression ratio generally tends to become higher as the load decreases. For instance, Patent Document 1 discloses a technique for operating a variable compression ratio mechanism so as to control a compression ratio with reference to a compression ratio map in which the optimal target compression ratio has previously been defined using a load and rotation speed (rpm) of an internal Combustion engine as parameters. In one example of this technique, the mechanical compression ratio is continuously varied and controlled within the range of 10 to 16.
Patent Document 2 discloses a technique for controlling a vehicle internal combustion engine with a variable compression ratio mechanism so as to change a compression ratio in a direction that reduces a shock caused by inertia torque during speed change operation of a transmission.
Patent Document 1 does not specifically mention the relationship between the speed reduction ratio of the transmission and the control of the compression ratio. However, the technique of Patent Document 1 presents a problem of deterioration in drivability caused by a large change of the compression ratio in response to driver's accelerator pedal operation (equivalently, demand load) when the vehicle is driven with fine adjustment of vehicle speed in a high speed reduction ratio state. In particular, the variable compression ratio mechanism using mechanical workings unavoidably causes a delay in operation response so that changes of the actual compression ratio are delayed relative to quick changes of the demand load. This leads to the occurrence of a torque change at driver's unintended timing, which can result in drivability deterioration.
Patent Document 2 does not give any solution to the above drivability problem based on the speed reduction ratio of the transmission.